Semiconductor light emitters are well known in the art. Semiconductor light emitters include light emitting diodes (LEDs) and semiconductor lasers. LEDs are devices of choice for many display applications because of advantages LEDs possess over other light sources. These advantages include a single relatively small size, a low operating current, a naturally colored light, a low power consumption, a long life, a maintained high efficiency (power in versus light output), an acceptable dispersal of light, and a relatively low cost of manufacture compared to other light sources.
Applications for LEDs include the replacement of light sources, such as incandescent lamps, especially where a colored light source is needed. LEDs are often used as display lights, warning lights and indicating lights. This colored light source application arises from a LED emitting radiation that produces an inherently colored light. The color of light emitted by a LED is dependent on the type of semiconductor material relied upon and its physical characteristics. The LED has not been acceptable for lighting uses where a bright white light is needed, due to the inherent color.
LEDs rely on its semiconductor to emit light. The light is emitted as a result of electronic excitation of the semiconductor material. As radiation (energy) strikes atoms of the semiconductor material, an electron of an atom is excited and jumps to an excited (higher) energy state. The higher and lower energy states in semiconductor light emitters are characterized as the conduction band and the valence band, respectively. The electron, as it returns to its un-excited (lower) energy state, emits a photon. The photon corresponds to an energy difference between the excited state and lower energy state, and results in an emission of radiation. The methods for exciting electrons vary for semiconductor light emitters, however, one method is excitation by the well-known method of injection electroluminescence.
Semiconductors are generally classified into three types, p-type, n-type and intrinsic semiconductors. Intrinsic semiconductors comprise either p-type or n-type semiconductors, and are formed by introducing impurities (dopants) of p-type (donor) or n-type (acceptor), respectively. In an n-type semiconductor, electron conduction (negative charge) exceeds acceptor hole (absence of electrons) concentration and electronic conduction is by donor electrons. In a p-type semiconductor, the hole concentration exceeds the electrons, and conduction is by acceptor holes.
Semiconductor light emitting devices are essentially characterized by p-type material and n-type material having a pn-junction there between or within p-type and n-type material regions. At equilibrium, no light is emitted by the semiconductor light emitting device. If electrons from the n-type material are coaxed into the conduction band over holes of the p-type material, electrons are excited. Electrons, once excited, will relax from their excited energy level at the conduction band to the valence band. The relaxation results in radiation (photon) emission. The radiation is normally ultraviolet radiation with about a 370 nm wavelength, which is not visible to the human eye. Radiation to be visible light must possess a wavelength within the visible spectrum. Phosphors are commonly used to convert non-visible radiation into visible radiation.
The use of phosphors has been attempted for converting LED radiation into visible light. LEDs, which include one or more phosphors for converting radiation into a "white" light merely produce a yellow-whitish light. For example, one yellow-whitish light emitting LED comprises a blue-light emitting LED, which has an emission wavelength equal to about 450 nm, provided with a yellow-light emitting phosphor, such as for example Y.sub.3 Al.sub.5 O.sub.12 --Ce.sup.3+, (YAG-Ce). The yellow-light emitting phosphor absorbs radiation emitted from the LED, and converts the absorbed radiation to a yellow-whitish light. The yellow-whitish light of the above example is produced with a relatively low efficiency (energy in versus energy out), for example only in a range between about 60% to about 80%. The yellow-whitish light, while suitable for limited applications, fails in applications where a true bright white light of high intensity and brightness is desired.
Color temperature is a standard used to compare the color of various light, for example fluorescent, incandescent, and other light types. Color temperature is related to a temperature of a black body that would give an equivalent tone of white light. In general, a lower color temperature is indicative of a redder tone of the white light. Conversely, a higher color temperature is indicative of a bluer tone of white light. There is no individual specific lamp component having a temperature equal to or determinative of the color temperature. For the yellow-whitish light described above, the color temperature falls in a range between about 6000 Kelvin to about 8000 Kelvin, with a resultant a color rendering index (CRI) less than about 85. The lumens per watt (LPW) of the above-described LED are in a range of about 5 LPW to about 10 LPW.
LED radiation at about 5 LPW to about 10 LPW with a CRI less than about 85 is not acceptable for lighting applications. Most lighting applications require a LPW that is at least 15 LPW, with a CRI maintained at or above 85, to increase the light source efficiency. Further, known LED light sources do not provide a single LED with a sufficient LPW and CRI for most generalized lighting applications, especially for white light.
Blue and green light emitting LEDs have been difficult to develop, even though acceptable yellow and red light emitting LEDs are well-known. Green and blue light emitting LEDs are limited because they possess several undesirable shortcomings. These green and blue LEDs possess a low intensity, brightness, and power, so their lights are not sharp and bright, nor are they high-quality (possessing wavelengths at or near the mid-point of the wavelength range for a particular color). Further, their emitted lights are not tuned at wavelengths to be combined with other light sources, such as red, to provide a bright white light. Accordingly, LED usage is severely limited, and not usable in place of compact fluorescent lamps, emergency lights, exit signs and the like.
Therefore, a single source LED that produces a bright white light is needed. Also, a blue-light emitting LED and a green-light emitting phosphors are needed for phosphor conversion material blends to produce a bright white light.